Five Challenges Faced In Planning Basement Construction
It is important to strike the right balance and hire a knowledgeable and experienced engineer with specific knowledge of London basement construction, to get the best price and outcome. That is what ‘value engineering’ is really about.
2. The Risk Of Losing Space
One of the possible risks of over-engineering is that it can often result in the loss of significant potential usable space, whereas experienced value engineering might lead to more usable space and an increase to the property’s value by square feet.
As a recent example, a client Knowles worked with had previously approached a less experienced construction company to plan a basement construction. The original engineer had planned to add an internal liner reinforced wall using underpinning, plus a reinforced wall. Although a combination of underpinning and a reinforced wall may sometimes be necessary, it wasn’t in this instance.
The property in question is located in a geographical area where space equates to £1,000 per square foot, so to go ahead with this plan would have resulted in a £130,000 lost potential and added a further £50,000 in unnecessary materials and labour. On top of that, an additional £30,000 saving was made to the final project cost with the omission of the RC liner wall and reduced temporary works.
Using the philosophy of ‘value engineering’ can help to significantly reduce costs for the customer from the very start of the project and therefore early involvement by the proposed contractor may save a lot of money for the project.
3. Underpinning Safety
Underpinning -the extension of the building’s foundations or load bearing walls downwards to create, not only the basement walls but also the new building foundations- is, of course, an important part of the process not just for the project property, but also for neighbouring properties.
The structural engineer will usually have drawn up neat plans based on straight lines, but it is only when you get down into the basement area for underpinning that you see it’s not straight and even – it’s messy, with layers of mud, water and clay. It’s not like the neat dotted lines on the plan.
It is very important to ensure that even any temporary works that may be required to support the building, prior to, or during the main works, are planned and carried out correctly. Otherwise, the building may also be unsafe and increase the likelihood of damage to neighbouring structures. In order to meet the highest standards of safety in basement design and construction, underpinning must be carried out perfectly, particularly with double or triple storey basements where there may be a pool or even an elevator for vehicle use.
4. Site Batched Or Pumped Concrete
Many structural engineers don’t like using site batched concrete for construction projects as it can sometimes be less accurate than concrete that has been ready-mixed in a processed environment, although cube testing can be used.
However, if measured correctly, site batched concrete is usually the best option for a basement construction project. The main reason for this is to keep the site traffic and any disruptions to neighbouring properties to a minimum, as basement construction projects often take place in residential neighbourhoods. Site batching eliminates the requirements for a truck to pump large volumes of concrete.
Often, during underpinning, only 2-3 cubic metres of concrete are required at a time anyway, so there is also no need to have a large amount of pumped concrete sent to the site.
Knowles is using specially designed trucks that pump the concrete at low pressure, like a toothpaste tube. Permanent rubber hoses can be set up to enable a fast and efficient delivery of 2-6 cubic metre batches within 30 minutes.
5. Considering Damp Issues – Water Proofing Methods
A key consideration in the design and construction process is the lifespan and preventing future problems such as damp.
Choosing the correct waterproofing method according to the site characteristics and design requirements will prevent groundwater from seeping into the basement over many years and causing damp, as well as preventing structural damage from excessive hydrostatic pressure.
There are two main methods of waterproofing. ‘Tanking’ uses cement based renders and slurries and ‘cavity drain membranes’ are sheets of Polyethylene that cover the structure like a studded sheet, keeping moisture out and letting it flow into underground sumps and drain away from the building.
Waterproofing does not have to be carried out at the same time as the construction. Knowles’ preferred method is to carry out the structural works (such as RC underpinning, RC retaining walls and floor slab) initially, with only additives sometimes being used during this time. Good preparation of the abutting surfaces and vibrating of the concrete during pouring will consolidate the concrete and make it less porous.
Water penetration to the concrete structure is observed thereafter. Water seepage will normally only occur on the construction joints, which can then be treated if apparent.
In the event of damp occurring, moisture enters through the joints rather than through the concrete itself. In order to make the surface less porous, poured concrete is vibrated to consolidate it.
Knowles has over 25 years of London basement construction, both new and retrofitted, always encompassing the notion of value engineering. For more information call 01344 886 898, or email.